Regular readers may remember I managed to break the blades of our small, student-built wind turbine, necessitating new blades, and providing an opportunity for a bench test of the alternator.
This is a vehicular alternator fitted with a permanent magnet instead of an electromagnet for a rotor, significantly changing the power production curve. A regular alternator for a car produces a relatively constant voltage output using a feedback loop involving an electromagnetic rotor. As the demand for power changes, a voltage regulator supplies more power to the rotor, making a stronger magnet, increasing the current output, or amperage. Voltage stays more or less constant at about 13.5, but amps increase, to power more peripheral devices (headlights, wipers, battery charging, etc). The constant voltage is particularly good for electronic devices, such as your car's computer. Electronic devices like regular power supply.
Batteries, on the other hand, such as the six golf cart batteries supplied by this alternator that power the "Eco-cottage" where the students in these photos live, can soak up a wider range of current, and a more varied range of voltage. Our batteries need a bit more than 12 volts to start recharging. Up to a point, the more current they get, the faster they recharge.
With the permanent magnet rotor, both voltage and current increase with RPM. RPM is obviously related to wind speed. The charge controller in this domestic DC system can manage the output of this turbine with no operating problems up to about a 35 mph wind speed.
In our bench test, we used a motor to drive the turbine with a pulley. We wanted to check that the alternator was working, by detecting a voltage suitable for charging the batteries at an RPM that wind speeds at the turbine tower site could reasonably supply. We got this. We also wanted to find some idea of the variability of power output with wind speed, so we used different size pulleys to adjust the RPM. We used a witness mark on the pulley and another on the fan belt, and a watch, to determine RPM. We repeated the experiment using a different pulley. The first pulley providing an RPM of 716, for a voltage of 20.2, the other provided an RPM of 358, for a voltage of 9.9.
A simple extrapolation of the equation provided by these data points (which is an equation for a straight line of the form y = mx + b, where slope is 0.0283 and intercept -0.22) tells us that the required charging voltage of 12.5 is provided at an RPM of 449. We estimate we achieve this voltage at about 15 mph wind speed, but we will fit one of our older NRG anenometers to the tower to be sure this time. (This almost 20-year old equipment was freed up from other uses thanks to the donation of a brand new set of gear from NRG.)
Having done all this, we put the new blades on the turbine and got the whole thing ready for reassembling to the turbine tower, and then, as they say in Yorkshire, "t'jobs a good un" and it's quitting time.
If you're a high school or college teacher who would like to know how to make a small wind turbine like this, you can go to my webpage to download a PowerPoint slide show with instructions.
We'd like to thank Hydrogen Appliances for providing us with a new set of wind turbine blades for this project.
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